Bonding scheme for diamond components which has low thermal barrier resistance in high power density applications

ABSTRACT

A semiconductor device comprising:
         a semiconductor component;   a diamond heat spreader; and   a metal bond,   wherein the semiconductor component is bonded to the diamond heat spreader via the metal bond,   wherein the metal bond comprises a layer of chromium bonded to the diamond heat spreader and a further metal layer disposed between the layer of chromium and the semiconductor component, and   wherein the semiconductor component is configured to operate at an areal power density of at least 1 kW/cm 2  and/or a linear power density of at least 1 W/mm.

FIELD OF INVENTION

The present invention relates to a bonding scheme for diamond componentswhich has low thermal barrier resistance at high power densities.Certain embodiments relate to the bonding of high power densitysemiconductor components to a diamond heat spreader. Certain furtherembodiments relate to a metallized diamond heat spreader suitable forsuch applications.

BACKGROUND OF INVENTION

The promised performance of wide band gap electronic devices (e.g. GaN,SiC, and GaAs based devices) will result in much high power dissipationand localized heat generation at contacts and in channel regions thancan be accommodated by current state-of-the-art thermal managementconfigurations. As a consequence, use of conventional cooling techniquesimposes a ceiling on wide band gap device performance and reliability.Overcoming such barriers requires thermal engineering at the macro,micro, and nano-scale, which can provide significant reductions in thenear-junction temperature rise and component thermal resistance.

Specific challenges relate to heat spreading in certain types of radiofrequency (rf) power devices. In such devices the local power densitiescan exceed, for example, 1 MW/cm². Spreading this heat and lowering thejunction temperature enables increased reliability and also continuouswave performance. In addition to electronic device applications, thereis also a need to improve upon current state-of-the-art thermalmanagement configurations in certain extreme optical applications.

Synthetic diamond materials have been proposed as an ideal solution inextreme thermal management applications due to the high in-plane thermalconductivity of such materials. For example, various grades of syntheticdiamond material grown by chemical vapour deposition (CVD) are alreadycommercially available for thermal heat spreading applications includingboth polycrystalline and single crystal synthetic diamond materials.

One problem with using diamond materials in such application is thatdiamond materials can be difficult to bond to other components and thisis a particular issue when used in high power density applications whereany bonding will be subject to large changes in temperature and wherethe thermal barrier resistance becomes critical to device performance.Metallization of diamond heat-spreaders is required to provide awettable surface for die-attachment. While diamond is known to be anexcellent room temperature heat spreader, its usefulness in reduction ofdevice junction temperatures is reduced by the TBR (thermal barrierresistance) associated with its method of attachment to electricaldevices.

Typically, for reasons of adhesion and mechanical robustness,three-layer metallization schemes are used for bonding of diamondcomponents. An example of such a three-layer metallization schemecomprises: (i) a carbide forming metal layer which forms a carbidebonding to the diamond component; (ii) a diffusion barrier metal layerdisposed over the carbide forming metal layer; and (iii) a surface metalbonding layer disposed over the diffusion barrier metal which providesboth a protective layer and a solderable/wettable surface layer ontowhich a metal solder or metal braze can be applied to bond the diamondcomponent to another device component. A particular example of such athree-layer metallization scheme is Ti/Pt/Au and typically diamond heatspreading components are sold in metallized form coated with such athree-layer metallization structure such that other components can readybe mounted and bonded to the diamond component using a solder or braze.

In general, as well as mechanical attachment, the layered metallizationcoating should be consistent with low thermal barrier resistance, suchas to maximize the effectiveness of a diamond heat spreader bonded to ahigh power density semiconductor component.

Across a diamond-metal-semiconductor interface in such an application,heat transport is a non-trivial physics process, alternating betweenphonon and electron transport processes with different scatteringmechanisms. Despite the actual thickness of some of the formedinterfaces being very low, for example the titanium layer forming atitanium carbide bond with the diamond surface can be very thin (onlytens of nanometers), these interfaces can add considerably to thethermal resistance and hence effectiveness of the thermal solution andthis is particularly problematic in high power density applications.

Following on from the above, the present inventors have revisited theproblem of mounting and bonding diamond components with the aim ofproviding a bonding methodology which has improved functionalperformance in terms of lower thermal barrier resistance when used inhigh power density applications.

SUMMARY OF INVENTION

The effect of thermal resistance scales with increased power density.The present inventors have recognized that a class of devices with powerdensities of at least 1 kW/cm² benefit substantially in terms of reducedjunction temperature for a given power density, if the standardtitanium-platinum based metallization scheme is replaced with analternative chromium based metallization scheme.

One aspect of the present invention provides a semiconductor devicecomprising:

-   -   a semiconductor component;    -   a diamond heat spreader; and    -   a metal bond,    -   wherein the semiconductor component is bonded (i.e. adhered) to        the diamond heat spreader via the metal bond,    -   wherein the metal bond comprises a layer of chromium bonded        (i.e. adhered) to the diamond heat spreader and a further metal        layer disposed between the layer of chromium and the        semiconductor component, and    -   wherein the semiconductor component is configured to operate at        an areal power density of at least 1 kW/cm² and/or a linear        power density of at least 1 W/mm.

The metal bond may also comprise a solder or braze. In this case, thefurther metal layer can be selected to be a metal which provides asolderable/wettable surface layer onto which a metal solder or metalbraze can be applied to bond the diamond heat spreader to thesemiconductor component. An example of such a metal is gold. As such,the bond may comprise a chromium layer bonded to the diamond heatspreader, a gold layer over the chromium layer, and a metal solder ormetal braze, such as a tin-based solder, disposed between the gold layerand the semiconductor component. For example, a chromium and goldmetallization coating may be provided on the diamond component and thiscan then be soldered or braze bonded to the semiconductor componentusing a tin-based solder. The semiconductor component may itself have ametallization coating which will be discernible in the final bond as ametal layer disposed between the solder or braze and the semiconductorcomponent. However, it will also be noted that during the bondingprocess there will be some intermixing of the solder or braze metal andthe metallization coatings on the diamond and semiconductor components.For example, when a Cr/Au metallization is provided on the diamondcomponent and a tin solder is used to bond the diamond component to thesemiconductor component then the final bond will comprise a chromiumlayer bonded to the diamond (forming a chromium carbide interface withthe diamond) and a mixed AuSn layer including gold-tin intermetalliccompounds.

Alternatively, if for example a diffusion bonding method is used ratherthan a solder or braze, then the bond may only consist of a chromiumlayer and a gold layer (or other suitable metal layer which can bediffusion bonded). For example, a chromium and gold metallizationcoating may be provided on the diamond component and a goldmetallization coating may be provided on the semiconductor component andthen the two gold layers can be diffusion bonded together.

According to another aspect of the present invention there is provided ametallized diamond heat spreader comprising:

-   -   a diamond heat spreader; and    -   a metal coating disposed on the diamond heat spreader,    -   wherein the metal coating comprises a layer of chromium bonded        to the diamond heat spreader and a further metal layer disposed        over the layer of chromium, and    -   wherein the diamond heat spreader has one or more of the        following characteristics:    -   a surface roughness Rq of no more than 1000 nm, 500 nm, 100 nm,        50 nm, 20 nm, 10 nm, or 5 nm;    -   a thermal conductivity of at least 1000 W/mK, 1300 W/mK, 1500        W/mK, 1800 W/mK, or 2000 W/mK;    -   a nitrogen concentration of less than 20 pμm;    -   a low or absent graphitic sp2 carbon signal as assessed by Raman        at an excitation wavelength of 633 nm; and    -   a Raman full width half maximum (FWHM) peak width of no more        than 20, 10, 8, 5, 3, 2, or 1.5 cm⁻¹.

For example, the metal coating may consist of a two-layer structureincluding a chromium layer and a layer of gold (or other suitable metalwhich can provide a solderable/wettable surface layer or a diffusionbond) as compared to the standard three-layer structure oftitanium-platinum-gold. In this case, the barrier layer of platinum canbe dispensed with as the chromium itself functions as an effectivediffusion barrier. This reduces the number of thermal barrier interfacesin the bond as well as reducing the formation of intermetallic compoundswhich increase thermal barrier resistance. When the diamond component isbonded to another component using a solder or braze then the final bondwill generally have the layer structure chromium/gold/solder.Alternatively, if the gold layer coating the diamond component isdiffusion bonded to another gold layer on the other device componentthen a solder or braze may be dispensed with.

The metallized diamond heat spreader as defined above can be used inhigh power density semiconductor devices. Furthermore, the metallizeddiamond heat spreader may also be used as an optical component in highpower optical devices. In the latter case, a low optical absorptiongrade of diamond material is selected for the diamond component suchthat both thermal and optical requirements are met for suchapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, embodiments of the present inventionwill now be described by way of example only with reference to theaccompanying drawings, in which:

FIGS. 1(a) and 1(b) show cross-sectional illustrations of asemiconductor component bonded to a diamond heat spreader using astandard Ti/Pt metallization scheme and an alternative Cr metallizationscheme respectively;

FIG. 2(a) shows four GaN-on-SiC samples, each attached to a 10×10 mm²diamond heat spreader;

FIG. 2(b) shows one of the samples in more detail illustrating that eachsample is mounted with several high electron mobility transistors(HEMTs), each having four fingers and different device lengths;

FIG. 3 shows an experimental set-up in which energy dissipation iscompletely controlled to keep the same conditions for all the samplesand enable a direct comparison between samples with T₁ and T₂ identicalin all the samples;

FIG. 4 shows a single HEMT structure illustrating three furthertemperature measurements T₃ to T₅ in addition to T₁ and T₂ illustratedin FIG. 3—the power profiles of the five temperatures T₁ to T₅ solvedsimultaneously allows one to extract the unknown variables: (i) Gan/SiCthermal barrier resistance (TBR); (ii) SiC thermal conductivity (T_(c));(iii) solder layer thermal conductivity (T_(c)); (iv) sample/diamond(TM180) thermal resistance; and (v) diamond(TM180)/temperature-controlled-stage thermal resistance;

FIG. 5 shows an example of a small device and an example of a largedevice—T₅ in small devices and T₃ in large devices (corresponding to thetemperature locations illustrated in FIG. 4) are very sensitive to thesolder thermal conductivity;

FIG. 6 shows a plot of temperature T₅ versus power comparing a devicecomprising a Ti/Pt based bond with a device comprising a Cr based bondand showing that as power is increased then the Cr based bond leads to asignificant reduction in the temperature T₅;

FIG. 7 shows a plot of temperature T₁ versus power comparing a devicecomprising a TiPt based bond with a device comprising a Cr based bondand showing that as power is increased then the Cr based bond leads to asignificant reduction in the temperature T₁;

FIG. 8 shows a plot of temperature versus X-profile comparing a veryhigh density 90 fingers GaN-on-SiC high performance analogue (HPA)device comprising a Ti/Pt based bond with the same device mounted usinga Cr based bond and showing that the Cr-based bond leads to asignificant reduction in the temperature across the X-profile of thedevice;

FIG. 9 shows a plot of the temperature ratio between the two devices ofFIG. 8 illustrating an approximately 25% reduction in temperature acrossthe device in moving from a Ti/Pt based bond to a Cr based bond;

FIG. 10 shows a plot of temperature versus Z-profile comparing a veryhigh power density GaN-on-SiC HPA device comprising a Ti/Pt based bondwith a device comprising a Cr based bond and showing that the Cr basedbond leads to a significant reduction in the temperature through theZ-profile of the device;

FIG. 11 shows a plot of thermal conductivity for different bondingsolutions illustrating that the thermal conductivity of a solder layerwith the new Cr metallization is 4-times higher than the standardthermal conductivity of this layer with a Ti/Pt metallization scheme;and

FIG. 12 shows a plot of the difference in channel temperature between adevice comprising the standard Ti/PT metallization versus the Crmetallization for two different GaN-on-SiC devices analysed, a smalldevice with a very localised heat generation and a very high powerGaN-on-SiC HPA, showing the improvement achieved in the reduction of thechannel temperature: 25% for large HPAs and 12% for small devices—thisresult summarizes the huge improvement when the Cr metallization isapplied in devices which require very high power dissipation.

DETAILED DESCRIPTION

As described in the summary of invention section, one aspect of thepresent invention is to provide a high power density semiconductorcomponent bonded to a diamond heat spreader via a chromium-based metalbonding scheme. It has been found that for high power densityapplications a chromium-based bonding scheme, e.g. Cr/Au, yields a muchlower thermal barrier resistance leading to much lower junctiontemperatures when operating at such high power densities compared withthe standard titanium-platinum bonding solution, e.g. Ti/Pt/Au.

The diamond component can be provided with a two layer metallizationstructure including a layer of chromium bonded to the diamond componentforming chromium carbide and metal layer, such as gold, silver,tantalum, or tin, disposed over the chromium layer and providing asurface layer which is suitable for bonding via a solder, a braze, orvia diffusion boding to another component. For example, the second metallayer may be a gold layer or may comprise gold and tin. In oneconfiguration the metallization coating on the diamond is Cr/Au and atin solder is used to bond the coated diamond component to anothercomponent such as a semiconductor die.

The present invention is illustrated herein by comparing two differenttypes of bonding structure:

-   -   (i) a device structure which utilizes a standard        titanium-platinum-gold metallized diamond heat spreader and a        tin-based solder to bond the semiconductor component to the        metallized diamond heat spreader such that the bonded device        structure comprises the layer        structure-diamond/Ti/Pt/AuSn/semiconductor component; and    -   (ii) a device structure which utilizes a chromium-gold        metallized diamond heat spreader and a tin-based solder to bond        the semiconductor component to the metallized diamond heat        spreader such that the bonded device structure comprises the        layer structure-diamond/Cr/AuSn/semiconductor component.

Chromium has previously been proposed as a possible bonding metal fordiamond materials as it is a carbide forming metal which can adhere todiamond materials by forming a carbide with a surface layer of carbon inthe diamond material. However, a titanium-platinum bonding solution hasbecome the standard due to advantages in terms of mechanical strength ofthe bonding. Furthermore, at standard power densities there is littledifference in thermal performance between chromium based bonding andtitanium-platinum based bonding. What the present inventors havesurprisingly found is that for very high power density applications achromium-based bonding scheme yields a much lower thermal barrierresistance leading to much lower junction temperatures when operating atsuch high power densities compared with the standard titanium-platinumbonding solution. As such, it has been found that for these very highpower density applications a chromium-based bonding scheme is preferredover a standard titanium-platinum bonding solution. While the thermaladvantages of using chromium at lower power densities are notsignificant, at very high power densities they become very significantand can out-weigh any mechanical advantages of using a standardtitanium-platinum bonding scheme. While not being bound by theory, thereason why a chromium bonding scheme has a lower thermal barrierresistance than a titanium-platinum bonding scheme is outlined below.

The standard Ti/Pt/Au metallization scheme is very reactive when appliedto soldering schemes such as a Sn-based solder, having some featureswhich can result in a poorer thermal management under high powerdissipation conditions. It is well established that the thermalconductivity of AuSn or AgSn solders are around 50-70 W/mK. However, theinteraction of these alloys with elements in the metallization canresult in a reduction from these values. When the Sn diffuses to the Aulayer it results in the formation of several intermetallic compoundslike Au5Sn, AuSn4, and others (Reliability and Failure of ElectronicMaterials and Devices By Milton Ohrin, M. T. Sheen Journal of electronicmaterials, 31, 8, 895, (2002)). These intermetallic formations canpromote the apparition of defects like Kirkendall voids or micro-crackswhich ultimately impact negatively on thermal transport. In the Ti/Pt/Aumetallization scheme, it has been demonstrated that the Sn can diffuseto the Pt-diffusion barrier, and ultimately even reach the Ti layer (X.Liu et al. Electronic Components and Technology Conference, 2004.Proceedings. 54th, vol. 1, no., pp. 798,806 Vol. 1, 1-4 Jun. 2004). Thisresults in the formation of different intermetallic compounds in thenear diamond interface region which has a strong impact on thermaltransport across this interface. For instance, the formation of TiPtSn,which has a resistivity of 0.51 μΩ/W, supresses electronic thermaltransport which is the most important thermal transport mechanism inmetals (T. T. M Palstra et al., Journal of Magnetism and MagneticMaterials 67 (1987) 331-342), while the formation of Pt_(x)Ti, PtTi_(y)and PtSn creates a region in which the lattice thermal conductivity isseverely disturbed by the disorder created by the existence of thesecompounds. Ultimately, it has been demonstrated that the Pt maycompletely reacts with the Sn, allowing the reaction of the Ti with theAu and Sn by forming Au—Sn—Ti compounds under typical device temperatureoperation (G. GHOSH Acta mater. 49 (2001) 2609-2624). Therefore, the useof the standard Ti/Pt/Au metallization scheme with a (Au/Ag)Sn solderintroduces a heavily disordered region in the near diamond interfacewhich ultimately introduces a thermal barrier which can be veryimportant when it comes to dissipate high energy densities. When asimple Cr/Au scheme is used instead of the Ti/Pt/Au, it has beendemonstrated that the Cr does not form any intermetallic compounds,being a very efficient diffusion barrier for Sn (X. Liu et al.Electronic Components and Technology Conference, 2004. Proceedings.54th, vol. 1, no., pp. 798,806 Vol. 1, 1-4 Jun. 2004). The use of Crtherefore enables thermal transport without adding thermal barriers,boosting thermal management under high power dissipation densities. Onthe other hand, the transference of energy from the diamond to othermaterials is not a trivial issue; this transference requires aphonon-phonon and phonon-electron coupling between materials whichdepends on the phonon density of states and scattering mechanisms acrossthe metal/diamond junction. This energy transference is related to thedifference in sound velocities and Debye temperatures in between thediamond and the metal, resulting in a high thermal boundary resistance(TBR) between very dissimilar materials. In this case, a diffusemismatch model indicates that the energy transference at a Cr/Diamondinterface is more than eight times higher than for a Ti/Diamondinterface. As a matter of fact, the Cr/Diamond interface has beendemonstrated to be one of the most efficient interfaces for thetransference of heat from/to the diamond (Monachon and Weber, EmergingMaterials Research, Volume 1 Issue EMR2, 89-98 (2012)), for instancehaving being used for boosting the thermal transference in Cu/Diamondmatrix compounds when the diamond is coated with Cr (K. Chu et al.Journal of Alloys and Compounds 490 (2010) 453-458). Effectively, thechromium based bonding scheme has better phonon compatibility whencompared to the standard titanium-platinum bonding. That is, transfer ofheat via electron flow in the metal bond to phonon propagation in thediamond material is improved when using chromium as opposed totitanium-platinum.

Following on from the above, the present inventors have also realizedthat the efficiency of heat transfer between electron flow in thechromium metal and phonon propagation in the diamond heat spreader willbe dependent on the surface structure of the diamond material and thematerial properties of the diamond, particular at and near the surfaceof the diamond heat spreader which forms an interface with the chromiummetal. For example, different grades of diamond material which havedifferent grain sizes, sp2 carbon content, extended crystallographicdefects such as dislocations, and point defect concentrations such asnitrogen-based point defects can also impact on heat transfer across thediamond metal boundary. As such, tailoring the diamond defect structureby changing the aforementioned variables, can be used to optimize phononproperties and can lead to an improved metallized diamond heat spreaderproduct for use in high power density applications

In addition to the types of materials used to bond diamond to anothercomponent, the layer thicknesses of the materials also affects thethermal barrier resistance of the bond. For example, the layer ofchromium bonded to the diamond heat spreader may have a thickness in arange 5 nm to 500 nm or 50 nm to 250 nm. A further metal layer, such asgold, disposed on the chromium may have a layer thickness in a range 100nm to 1 micrometer or 300 nm to 700 nm. The layer of metal disposed onthe chromium may have a thickness in a range 5 μm to 100 μm or 15 μm to40 μm when a solder or braze is applied, i.e. including the solder orbraze.

As previously described, the advantageous technical effect of utilizingchromium instead of titanium-platinum is observed when operating at highpower densities. The equivalent CW areal power density of thesemiconductor component may be at least 1 kW/cm², 2 kW/cm², 5 kW/cm², 10kW/cm², 20 kW/cm², 50 kW/cm² or 100 kW/cm², 1 MW/cm², 2 MW/cm², 4MW/cm², 6 MW/cm², 8 MW/cm², or 10 MW/cm². Alternatively, oradditionally, the linear power density of the semiconductor componentmay be at least 1 W/mm, 2 W/mm, 2.5 W/mm, 3 W/mm, 4 W/mm, 6 W/mm, 8W/mm, or 10 W/mm. It has been found that in general the larger the powerdensity the larger the benefits of using a chromium-based bondingsolution instead of a Ti/Pt based bonding scheme. The most suitablepower density definition will depend on the type of semiconductordevice. For high power density RF devices the power density is usuallydefined as a linear power density in terms of Watts per unit gate width.However, for other devices such as laser diodes, light emitting diodes,power switches and microprocessors an areal power density measurement ismore appropriate. In the latter case it is the area of the active regionwhich is key, be it light emitting or current switched.

The present invention also provides a metallized diamond heat spreadercomprising:

-   -   a diamond heat spreader; and    -   a metal coating disposed on the diamond heat spreader,    -   wherein the metal coating comprises a layer of chromium bonded        to the diamond heat spreader and a further metal layer, such as        gold, disposed over the layer of chromium, and    -   wherein the diamond heat spreader has one or more of the        following characteristics:    -   a surface roughness Rq of no more than 1000 nm, 500 nm, 100 nm,        50 nm, 20 nm, 10 nm, or 5 nm;    -   a thermal conductivity of at least 1000 W/mK, 1300 W/mK, 1500        W/mK, 1800 W/mK, or 2000 W/mK;    -   a nitrogen concentration of less than 20 parts per million;    -   a low or absent graphitic sp2 carbon signal as assessed by Raman        spectroscopy at an excitation wavelength of 633 nm; and    -   a Raman full width half maximum (FWHM) peak width of no more        than 20, 10, 8, 5, 3, 2, or 1.5 cm⁻¹.

Such a metallized diamond heat spreader can be used in high powerdensity semiconductor devices as previously described. Furthermore, thediamond heat spreader may also be used as an optical component in highpower optical devices. In the latter case, a low optical absorptiongrade of diamond material is selected for the diamond component suchthat both thermal and optical requirements are met for suchapplications.

FIGS. 1(a) and 1(b) show schematic cross-sectional illustrations of asemiconductor component bonded to a diamond heat spreader using astandard Ti/Pt bonding scheme and a chromium-based bonding schemerespectively. FIG. 1(a) shows a device structure which utilizes astandard titanium-platinum metallization and comprises the layerstructure-diamond/Ti/Pt/AuSn/semiconductor component. FIG. 1(b) shows adevice structure which utilizes a chromium metallization and comprisesthe layer structure-diamond/Cr/AuSn/semiconductor component. These twobonding solutions have been used to show the benefits of using thechromium-based bonding scheme in high power density applications. Thelayer thicknesses for the metallization coatings on the diamond in eacharrangement are as follows: Ti/Pt/Au-100/120/500 nm; Cr/Au-100/500 nm. Atin-based solder having a thickness of 25 micrometers was used to bondthe metallized diamond components to semiconductor components.

FIG. 2(a) shows four HEMTs fabricated on GaN-on-SiC samples, eachattached to a 10×10 mm² diamond heat spreader. Each of the diamond heatspreaders is formed of polycrystalline diamond material from ElementSix. FIG. 2(b) shows one of the samples in more detail illustrating thateach sample is mounted with several HEMTs, each having four fingers anddifferent device lengths.

FIG. 3 shows an experimental set-up in which energy dissipation iscompletely controlled to keep the same conditions for all the samplesand enable a direct comparison between samples with T₁ and T₂ identicalin all the samples. FIG. 4 shows a single HEMT structure illustratingthree further temperature measurements T₃ to T₅ in addition to T₁ and T₂illustrated in FIG. 3. The power profiles of the five temperatures T₁ toT₅ solved simultaneously allows one to extract the unknown variables:(i) Gan/SiC TBR; (ii) SiC T_(c); (iii) solder layer T_(c); (iv)sample/diamond thermal resistance; and (v)diamond/temperature-controlled-stage thermal resistance.

FIG. 5 shows an example of a small device and an example of a longdevice. T₅ in small devices and T₃ in long devices (corresponding to thetemperature locations illustrated in FIG. 4) are very sensitive to thesolder thermal conductivity.

The thermal conductivity of the standard TiPtAu metallization is locatedin the average of the commercial devices tested with this metallizationscheme (13±3 W/mK). The new metallization scheme improves the thermalconductivity of the die-attach by a factor of three to four. In thesedevices the peak/channel temperature at normal operation is a ˜13% lowerwith the new chromium-based bonding scheme. The test devices are smalltransistors (4 fingers, 100-275 μm gate width). The temperature in thesesmall devices is mainly controlled by the thermal management of the die,not by the package which the diamond heat spreader is mounted. In thisregard, FIG. 6 shows a plot of temperature T₅ versus power comparing adevice comprising a Ti/Pt based bond with a device comprising a Cr basedbond and showing that as power is increased then the Cr based bond leadsto a significant reduction in the temperature T₅. FIG. 7 shows a plot oftemperature T₁ versus power comparing a device comprising a Ti/Pt basedbond with a device comprising a Cr-based bond and showing that as poweris increased then the Cr-based bond leads to a significant reduction inthe temperature T₁. The results in FIG. 6 and the subsequent Figure arefor a Cr metallization and a comparative TiPt metallization both ofwhich have a 25 micrometer thick AuSn solder disposed thereon.

A real GaN-on-SiC HPA 90 fingers device operated at 50V and 1A mountedunder real conditions in a window-package has been simulated to test theimpact of the metallization approach described herein. FIG. 8 shows aplot of temperature versus X-profile comparing a device comprising aTi/Pt based bond with a device comprising a Cr based bond and showingthat the Cr-based bond leads to a significant reduction in thetemperature across the X-profile of the device. FIG. 9 shows a plot of atemperature difference between the two devices of FIG. 8 illustrating anapproximately 25% reduction in temperature across the device in movingfrom a Ti/Pt based bond to a Cr-based bond. FIG. 10 shows a plot oftemperature versus Z-profile comparing a device comprising a Ti/Pt basedbond with a device comprising a Cr-based bond and showing that theCr-based bond leads to a significant reduction in the temperaturethrough the Z-profile of the device.

FIG. 11 shows a plot of thermal conductivity for different bondingsolutions illustrating that the thermal conductivity of a solder layerwith the Cr-metallization described herein is four times higher than thestandard thermal conductivity of this layer with a Ti/Pt metallizationscheme.

FIG. 12 shows a plot of the difference in channel temperature between adevice comprising the standard Ti/PT metallization versus theCr-metallization describe herein for two different devices indicatingthat the temperature in the channel is between a 25% (big HPAs) and a12% (small devices) higher in devices with the standard metallizationscheme compared with the Cr-based metallization scheme.

These results indicate that while the impact on the thermal managementof the chromium-based metallization scheme may be negligible for lowerpower devices it becomes very significant for very high powersemiconductor devices.

It is also envisaged that the metal coated diamond heat spreader asdescribed herein may be used in other applications such as opticaldevices where both thermal and optical properties of the diamondmaterial may be utilized, e.g. a high power laser window. In a range ofapplications a mounted diamond component may be provided comprising ametal coated diamond heat spreader as described herein which is adheredto another component via a solder or braze bond or otherwise adhered toanother device component without any solder or braze, e.g. via pressurebonding. However, the bonding solution as described herein has beenfound to be particularly effective in very high power semiconductordevice applications.

While this invention has been particularly shown and described withreference to embodiments, it will be understood by those skilled in theart that various changes in form and detail may be made withoutdeparting from the scope of the invention as defined by the appendingclaims.

The invention claimed is:
 1. A semiconductor device comprising: asemiconductor component; a diamond heat spreader; and a metal bond,wherein the semiconductor component is bonded to the diamond heatspreader via the metal bond, wherein the metal bond comprises a layer ofchromium directly bonded to the diamond heat spreader and a furthermetal layer disposed between the layer of chromium and the semiconductorcomponent, and wherein the semiconductor component is configured tooperate at an areal power density of at least 1 kW/cm² and/or a linearpower density of at least 1 W/mm.
 2. A semiconductor device according toclaim 1, wherein the layer of chromium has a thickness in a range 5 to500 nm.
 3. A semiconductor according to claim 1, wherein the furthermetal layer comprises one or more of gold, silver, tantalum and tin. 4.A semiconductor device according to claim 1, wherein the further metallayer has a thickness in a range of 5 to 100 μm including a solder orbraze material.
 5. A semiconductor device according to claim 4, whereinthe further metal layer has a thickness in a range 15 to 40 μm includingthe solder or braze material.
 6. A semiconductor device according toclaim 1, wherein the areal power density of the semiconductor componentis at least 2 kW/cm².
 7. A semiconductor device according to claim 1,wherein the linear power density of the semiconductor component is atleast 2 W/mm.
 8. A semiconductor device according to claim 1, whereinthe diamond heat spreader has one or more of the followingcharacteristics: a surface roughness Rq of no more than 1000 nm; athermal conductivity of at least 1000 W/mK; a nitrogen concentration ofless than 20 parts per million; a low or absent graphitic sp2 carbonsignal as assessed by Raman spectroscopy at an excitation wavelength of633 nm; and a Raman full width half maximum (FWHM) peak width of no morethan 20 cm⁻¹.
 9. A metallized diamond heat spreader comprising: adiamond heat spreader; and a metal coating disposed on the diamond heatspreader, wherein the metal coating comprises a layer of chromiumdirectly bonded to the diamond heat spreader and a further metal layerdisposed over the layer of chromium, and wherein the diamond heatspreader has one or more of the following characteristics: a surfaceroughness Rq of no more than 1000 nm; a thermal conductivity of at least1000 W/mK; a nitrogen concentration of less than 20 parts per million; alow or absent graphitic sp2 carbon signal as assessed by Ramanspectroscopy at an excitation wavelength of 633 nm; and a Raman fullwidth half maximum (FWHM) peak width of no more than 20 cm⁻¹.
 10. Ametallized diamond heat spreader according to claim 9, wherein the layerof chromium has a thickness in a range 5 to 500 nm.
 11. A metallizeddiamond heat spreader according to claim 9, wherein the further metallayer is selected from any of gold, silver, tantalum, and tin.
 12. Ametallized diamond heat spreader according to claim 9, wherein thefurther metal layer has a thickness in a range of 100 nm to 1micrometer.
 13. A metallized diamond heat spreader claim 9, wherein themetal coating is a two-layer coating consisting only of the layer ofchromium and the further metal layer.
 14. An optical componentcomprising a mounted diamond component comprising the metallized diamondheat spreader according to claim 9 adhered to another device.
 15. Asemiconductor component comprising the mounted diamond componentcomprising the metallized diamond heat spreader according to claim 9adhered to another device.